The pressing need for advanced battery technologies constitutes the driving force in developing new electrode formulations to replace conventional intercalation compounds and carbonaceous materials in current lithium-ion batteries and supercapacitors. Electrochemically active metals and metalloids that can form intermetallic alloys with lithium, such as silicon, germanium, and tin, as well as transition metal oxides that can react with lithium ions reversibly via conversion reactions, such as tin dioxide, iron oxide, and manganese dioxide, have great potential to radically boost the energy density of lithium-ion batteries. Nevertheless, despite their promise as electrode materials, these materials generally have relatively low electrical conductivities and also suffer from enormous volumetric expansion/contraction dynamics during charge/discharge cycling as a result of the lithiation/de-lithiation processes. These large volumetric changes often result in the pulverization of the electrode materials. Once fragmented in this manner, side reactions may then occur at the freshly formed electrode/electrolyte interfaces, and the electrode fragments may become isolated by the newly formed side products and lose electrical contact. These unwanted side reactions also gradually deplete the available electrolyte, and severely hinder the rate capability and deep cycling ability of the electrodes. Device performance and lifespan are thereby limited.
Attempts to address these deficiencies have included surrounding active nanomaterials with carbonaceous shells. In one approach, for example, carbon layers have been deposited onto active nanowires via the calcination of organic carbon precursors. In another approach, active nanoparticles have been inserted into carbon nanotubes. Nevertheless, in the former approach, the carbon coatings have tended to be highly defective and, as a result, have tended to exhibit low electrical conductivities. Moreover, in both approaches, the carbonaceous shells have tended to lose contact with the active materials or to rupture during cycling due to the stiffness of the shells. Once the carbonaceous shells are so compromised, the active materials are again exposed to unwanted side reactions with the electrolyte.
For the foregoing reasons, there is a need for alternative electrode technologies for use in high-performance energy storage devices such as batteries and supercapacitors that do not suffer from the several disadvantages described above.